Add user/company enable functionnality

We now provide a tool to re-enable users and companies.
Enlight the disabled companies

Add a orange box near to the name indicating those are disabled.
Add company enabling functionnality

Add:
* an item in the action menu
* a view to enable a company
The view is quite flexible to allow using the same code from the user
view.
Add user-enabling functionnality

Add:
* A 'display disabled' button in the action menu (and query filters)
* A view to enable a user
Now we can list disabled users and activate them.
Chek login of disabled account

When validating a user login in add/edit forms, we check non active
accounts to avoid sql integrity erros.
Merge branch 'enable_account'

The user directory now allows to list disabled users and to enable them
again
Fix docstring, super call and params
Add an export link to the customer list
Rename the customer to csv view
Configuration of the invoice prefix

The invoice prefix allow to mix two autonomie instances in the same
treasury.
Filter the prefix in sage export
Invoice number search is not a number anymore
Filter the prefix in the invoice search form

Since the number is stored as an int in the database, we remove the
prefix to query the invoices
Add the prefix on cancelinvoice generation
Add the prefix in the templates
Add the prefix to the sage export
Merge branch 'prefix_facture'

Fix #5 : Add an optionnal (and configurable) prefix to the invoice numbers